The use of wireless communication devices such as telephones, pagers, personal digital assistants, laptop computers, etc., hereinafter referred to collectively as “mobile appliances” or “mobiles stations” has become prevalent in today's society. In recent years, at the urging of public safety groups, there has been increased interest in technology which can determine the geographic position or “geo-locate” a mobile station in certain circumstances.
In the United States, mobile station locating equipment is being deployed for the purpose of locating wireless callers who dial 911. Other services in addition to emergency call servicing are contemplated and are referred to as location based services (LBS). Wireless location equipment is typically employed as an overlay to wireless communication networks, thus forming a network overlay geo-location system.
FIG. 1 illustrates a wireless communications network (1), which typically includes a plurality of base stations (BS) 10a, 10b, 10c and 10d (10(a-d)) a mobile station (MS) 20, and a mobile switching center (MSC) 30, also known as a base station controller (BSC). The wireless communication network allows communication between the MS and the BS over an air interface as is well known in the art.
The network overlay geo-location system is designed to work with the wireless communications network, and is made up of a plurality of local measurement units (LMU) 15, also known as wireless location sensors (WLS), a geo-location system controller (GCS) 35, also known as a serving mobile location center (SMLC) and a mobile positioning center (MPC) 45, also known as a gateway mobile location center (GMLC).
The LMUs 15 of the geo-location network overlay system are generally co-located at the wireless service provider BSs 10(a-d). The LMUs 15 conduct measurements on radio signal emitted by the MS 20 under control of the GCS 35. The GCS 35 is commanded by the MPC 45 to determine and report locations of specific MSs 20.
The GCS 35 is responsible for processing sensor information gathered from the LMUs 15. A database 55 stores the collected sensor information which is accessible by the GCS 35. When a location request is received, the mobile network provider will notify the GCS 35 as to which BS10(a-d) and/or respective BS sector (not shown) is serving a current call of a MS 20.
Most network overlay systems include a number of non-serving BS sites 10(b-d) located around the vicinity of the serving BS site 10(a), which are termed “secondary sites” and the corresponding sectors of the secondary sites are called “secondary sectors” (not shown). For this example, the secondary sites 10(b-d) are the sites located furthest from the MS 20, and the serving site 10(a) is located closest to the MS 20. The LMUs 15 co-located with the secondary sites 10(b-d) return location measurement information that enables the GCS 35 to compute the location of a caller/MS 20.
In operation, network overlay systems conduct measurements of radio frequency (RF) transmissions from the MS 20 and received by BSs 10(a-d), and estimate the location of the MS 20. The geographic location of the BSs 10(a-d) are known, and the location of the MS 20 with respect to the BSs 10(a-d) is represented by a reference position location which is an estimated location of the MS 20 that enables the geographic location of the MS 20 to be determined.
To geo-locate MS 20, one or more of the BSs 10(a-d) perform radio frequency measurements on the received signals transmitted from MS 20. Some techniques used to perform geo-location from signal measurements can include, for example: time of arrival (TOA), time difference of arrival (TDOA), uplink time difference of arrival (UTDOA), the angle of arrival (AOA), the signal power, or the unique/repeatable radio propagation path (radio fingerprinting). In addition, the geo-location systems can also use collateral information (e.g., locations of roads, dead-reckoning, topography, map matching etc.) to assist in geo-locating the MS 20.
UTDOA is a network-based geo-location algorithm that determines a MS's 20 position by calculating the difference in time required for a MS's 20 signal to arrive at different BSs 10(a-d). The arrival time measurements are performed by the LMUs 15 installed at selected BSs 10(a-d). UTDOA requires the target MS's 20 signal be measured by at least three LMUs 15. These measurements are sent to the GCS 35, which computes the location of the target MS 20 using the time difference of arrival of the RF signal transmitted from the MS 20 to the BSs 10(a-d).
AOA is another network-based geo-location algorithm that determines the MS's 20 position by establishing lines of bearing from the BSs 10(a-d) to the MS 20. AOA requires the installation of a LMU 15 at each AOA-equipped BS, and also requires direction-sensing antennas. The AOA algorithm searches for the target MS 20 on a serving sector of a serving BS10a, and on secondary sectors of secondary sites 10(b-d). The likelihood of obtaining an accurate location of the MS 20 using the AOA algorithm depends to a large extent on the correct choice of secondary sectors for the secondary sites 10(b-d). AOA is normally deployed in especially challenging geographic circumstances. For example, AOA is sometimes used in mountainous areas with sparse BS installations and challenging BS site-to-site geometry.
In addition to optimum selection of LMUs 15 at secondary site locations, it is important to have optimum LMU 15 coverage of the MS 20. Optimum coverage, or “good coverage”, illustrated in FIG. 1, would be realized by LMUs 15 surrounding the MS 20 from all sides and being physically close enough to receive accurate signal timing measurements. Geo-location accuracy can suffer from geometric dilution of precision, or “poor coverage” when the sensors are all located in roughly the same area and direction with respect to the MS 20, as illustrated in FIG. 2. Selecting a reference position as an estimated location of the MS 20 prior to selecting a secondary base station to task may provide optimum secondary site selection.
In one embodiment of applicant's disclosure, it may be desirable to select a secondary BS site to task by determining a reference position within the geographical area of the serving sector before selecting a secondary BS site. In another embodiment of applicant's disclosure, it may be desirable to select a secondary BS site to task by determining a reference position to be the location of the serving BS site. In yet another embodiment of applicant's disclosure, it may be desirable to select a secondary BS site to task based on a function of the reference position. In still another embodiment of applicant's disclosure, it may be desirable to select a secondary BS site to task based on a preferred parameter that cannot be produced by the other secondary sites.
In another embodiment of applicant's disclosure, it may be desirable to select a secondary BS site to task by computing a distance range to one or more secondary sites, omitting the secondary sites outside the distance range as potential secondary site candidates, computing the bearing angle from at least one secondary site to the reference position, determining which secondary sites do not have at least one sector beam that covers the reference position, omitting the secondary sites that do not have at least one sector beam that covers the reference position as potential secondary site candidates, and flagging the secondary site that is closest to the mobile station reference position as a selected secondary site.
Accurate geo-location performance is predicated on optimum selection and tasking of LMUs 15 at secondary sites 10(a-d) and their respective sectors. Optimum secondary site and/or secondary sector selection may provide superior geo-location performance results. Enabling the GCS 35 to select and task optimum secondary sites and/or secondary sectors may provide geo-location information necessary to estimate the location of the MS 20.
These and other advantages of the disclosed subject matter over the prior art will be readily apparent to one skilled in the art to which the disclosure pertains from a perusal of the claims, the appended drawings, and the following detailed description of the preferred embodiments.